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Comparizon of various energy systems and their roles
多様なエネルギー源の比較とその役割
Kunihiko Okano
岡野邦彦
Central Research Institute of Electric Power Industry,
2-11-1, Iwado-kita, Komae-shi, Tokyo 201-8511, Japan(財)電力中央研究所，〒201-8511 東京都狛江市岩戸北2-11-1
There are various requirements for future energy sources. Features of three options of advancedenergy technologies (solar/wind, solar power satellite and fusion) , which are expected to satisfy themost part of these requirements, are considered. Each option has various merits, but at the same time, noprospective energy option satisfies the requirements completely, i.e., every option has a certain degree ofdrawback(s). An optimized combination to make the best use of these technologies will be required.

1. Introduction
Based on recent resource survey [1], we canbelieve that the resource of fossil fuels and otherhydrocarbon resources will not run out during the21st century and possibly the next century, too. On
the other hand, a lot of reports have been pointed
out a possibility on the climate change in near
future due to the use of the fossil fuels. We need
new energy sources or advanced energy
technologies which emit less CO2 during operation,in order to reduce the CO2 emission rather than therestriction in resource of fossil fuel. Therefore, such
future energy system should be satisfy various
requirements, not only the abundance in resource.
energy supply 10 6 Oil 6ton
1次エネルギー生産量(10石油換算トン）10,000550ppm安定化ケース
550ppm case w/CO2(CO2処分あり、革新技術あり）
25,000

革新的技術
Adv.
technol

contr. & Adv. technol.

太陽光
solar
wind
風力

20,000hydro水力
biomassオマス
バイ

15,000fission
従来型原子力gas
天然ガス

10,0005,00002000 2010 2020

原油
petroleum

coal石炭2030

2040

2050 2060 2070

2080 2090 2100

西暦年
YearsFig. 1 Energy supply ant its breakdown

One of our good targets for the next 100 years is‘550ppm in CO2 concentration at 2100’, which wasshown by IPCC in 1995. An example of a typicalscenario of energy supply to achieve the above
target is shown in Fig.1 [based on the report ofwork by NEDO/RITE, 1998]. From such
perspective, we can see that any advanced energy
technology should be available, at latest, until themiddle of this century. If not, such energy
technology may not make any significant

contribution by 2100. Here we note that a period for
replacement of energy infrastructure by a newtechnology takes a lot of years, 20-30 years usually.
2. Options of advanced energy technology
We can consider the followings may be the major
requirements to be posed for the future energy
technology which will be used as basic powersources:
1. Abundant and widely available resources
2. Low emission of CO2 as well as other
undesirable substances and less waste to bedisposed
3. Acceptable cost with flexibility in plant scale
4. Stable power supply and less vulnerability tointernational affairs
5. Safety and security
In this review, we consider roles of following
three options of advanced energy technologies that
may satisfy the most part of these requirements;
1) Terrestrial solar and wind power plants
2) Solar Power Satellites (SPS) on
Geosynchronous Earth Orbit (GEO)
3) Fusion reactor
3. Characteristics of the options
3.1 Resource
Since the reserves of resource or available
energies is unlimited for any above options. But,
when we see them in practical conditions, there aresome issues. The available energy from solar power
and the available area for wind power depend on
the country [2]. Since the rectena (a power receiveron the earth) of SPS is large (60m2/GW) [3], theterrestrial area can be a restriction. Total power
generated by SPS will be limited by capacity of theGEO. The initial construction pace of fusion plants

will be restricted by the tritium breeding ratio[3].
3.2 Environmental Issue
The CO2 emission from all candidates is lowenough. However, concerning the disposability ofwaste, the solar/wind power plant may be the best.
On SPS, there is a concern on how to dispose asatellite (2x104 tons [4]) on GEO. The waste fromfusion is 2x104 tons throughout the plant life and it
is radioactive [5].
3.3 Cost and plant scale
Any energy plant should be provide the energy
within energy cost competitive with others. But, it
is also true that there are many unknown factors on
the cost of prospective energy, then, we do notdiscuss here the issue on the direct cost of plant.
Flexibility in plant scale (with acceptable cost) is
another issue. The solar/wind is very flexible inminimum plant scale. This is an important merit forsolar/wind plant. Although there is no inherent limit
for plant scale for SPS, a strong scale effect in cost
will exist because the size of antenna and rectena
seems to be determined by the beam divergencerather than the transmission power. Minimum scaleof magnetic confinement fusion will be restrictedby the inherent feature of fusion reaction. Large
power consumption at start- up can be another
weakness of fusion, especially in the area on poor
electric network. If laser fusion mitigates theseweakness, it may bring about an important breakthrough and increase the value of fusion energy.
3.4 Stability of power supply
Concept of stability includes two issues. The firstone is related to the inherent operational restriction.
The second issue is vulnerability of operation tointernational affairs.
For the fusion, there is no limit for continuousoperation, at least, in principle. The output of SPS
is interrupted due to eclipse by the earth before andafter the autumnal and spring equinox, but theduration of interruption is less than one hour perday. The solar/wind power plant is inherently
unstable. The cost to overcome this unstable feature
will be expensive. On the other hand, the solar/wind
power plant is highly invulnerable to international

affairs. The fusion will be invulnerable enough,because fusion fuel can be obtained from seawaterand the fuel cycle can be closed in an unit plant.
Since the GEO will be shared among foreigncountries, the SPS plants on the GEO might havesome vulnerability.
3.5 Safety and Security
The solar/wind can be regarded as having nohazard potential. In the case of SPS, there might besome unfavorable effect by electromagnetic wave
near the rectena. The power density on the rectenaof SPS is 23mW/cm2[6]. The fusion plants have ahazard potential due to associated radioactivity. The
bio-hazard potential of fusion is, however, lower by
1/1000 than the present fission plant[4].
4. Roles of the options
As shown above, each option has various merits aswell as weakness, and no option satisfies all therequirements completely. Therefore we have to usethem under the best-mix-policy. Some possibleroles for the options considered by the author isshown in Table 1. A path to the economical andecological future will be created by an optimized
combination among such advanced technologies.
References[1] H.H.Ronger, “An assessment of world hydrocarbon
resources”, Annu. Rev. Energy Environ. 1997, 22
217-62[2] K. Yamaji, K. Okada, K. Nagano et al., “WorldEnergy Resources: Endowments, Supply/Demand,Economics, and Related Technology Development”,
CRIEPI Report Y94001, Central Research Institute ofElectric Power Industry(CRIEPI), Tokyo, 1994
[3]Y. Asaoka et al., Fus. Technol. 39(2001)518-522.
[4] “New Paradigm of Energy Technology”,
under supervision of Y. Kaya, by Special Committeefor Investigation on Energy Issues in The InstituteElectrical Engineers of Japan, Ohmsha, (1995)[5]”Reports on Technical Feasibility of Fusion Energyand Extension of the Fusion Program and Basic
Supporting Researches”, Subcommittee of the Fusion
Council for Fusion development Strategy (2000)[6]Mitsubishi Research Institute, Inc., “Study of SpacePower Satellite System”, as a report of workcommissioned by NEDO, (1994)

Table 1 Characteristics of three options considered and possible roles for their best use
possible roles
issues for expansion into wide use
solar/wind effective & maximized use instability in the electric network
good condition area
SPS
base load use, H2 production,
limit in generating power by the GEO
usable for area on poor network
capacityFusionbase load use, H2 production,
limit in initial construction pace bysiting near consuming area
Tritium breeding